US Extended Forecast thru Aug 17

Flow Across The Lower 48 Is Fcst To Show Gradual Flattening Over The Course Of The Period As An Upr Low Near The Pac Nw Coast Weakens As It Drifts Ewd And A Vigorous System Weakens/Lifts Away From New England/Sern Canada By Next Weekend.

D+8 Multi-Day Means Maintain A Persistent Core Of Neg Hgt Anomalies Just S Of The Aleutians Within Bering Sea/Nrn Pac Troughing While They Indicate An Ewd Drift Of Pos Hgt Anomalies Assoc With A Ridge To The N Of Alaska. Even With The Motion Of The Latter Feature Teleconnections Still Agree Upon A Weakness Along / Just Offshore The West Coast With An Interior Wrn Conus Ridge/Ern Troughing Downstream.

Models/Ensembles Finally Appear To Be In The Process Of Converging Toward The Idea Of Short Term Nern Pac Energy Dropping Into The Northwest To Form A Rather Strong Upr Low. Given The Long Term Mean Ridge Expected Over The Interior West It Is Reasonable To See That Most Of The Latest Operational Runs Have Trended Less Amplified By The Time It Reaches The Rockies.

Beyond The Uncertainty With The Shape Of Nwrn Conus Energy As It Reaches The Nrn Plains Late In The Period... There Are Also Question Marks Over The Evolution Of One Or More Leading Shrtwv
Impulses With The Best Defined Feature Fcst To Be Over/Near Arizona As Of Early Wed.

Upstream The Ensemble Means Are Overall The Closest To The Teleconnection Favored Idea Of Maintaining A Weakness Near The West Coast. The Possibility That Some Mstr/Energy From Tropical Systems Julio And Genevieve May Become Embedded Within This Flow Will Also Have To Be Watched In Coming Days.

Most Guidance Remains Well Clustered For The System That Should Have Greatest Impact On The Northeast During The First Half Of The Period.

Sensible Weather Highlights...

The First Half Of The Period Should Be The Most Convectively Active Over The West... With Enhanced Clusters Of Showers/Storms Assoc With The Well Defined Upr System Initially Over The Northwest As Well As With Monsoonal Mstr And One Or More Impulses Aloft From Az/Ut Newd. Diurnal Activity Over Wrn Mexico May Extend Into Extreme Srn Az Through The Weekend Though.

From Thu Onward The Combination Of Mid Lvl Energy Reaching The Plains And Increasing Low Level Southerly Flow/Focusing Sfc Trough Should Increase The Potential For Convection Over Portions Of The Plains... But Guidance Spread Remains Sufficiently Great To Yield Low Confidence In Location / Timing.

Farther Ewd... New England Rnfl Enhanced By Dynamics Aloft And Low Lvl Onshore Flow Should Begin Tapering Off By Wed Night At Most Locations. The Front Trailing From New England Low Pressure Should Stall Over The Southeast And Provide A Focus For Rnfl Along The Gulf/Southeast Coasts And Over The Nrn Fl Peninsula.

Over The West The Unsettled Pattern During The First Half Of The Period Should Support Below Normal Daytime Temps From Wed Into Thu Or Fri With Some Moderation Expected By Next Weekend.

Meanwhile The Gradual Flattening Of Mean Flow Aloft Should Lead To Temps Closer To Normal Over The Ern Half Of The Conus By Next Weekend After This Area Experiences Multiple Days Of Minus 5-10f Anomalies. The Srn High Plains Should Be Consistently Above Normal By Fri-Sun.

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